Silicon chalcogenides

ABSTRACT

Binary, ternary and quaternary silicon chalcogenides of the formula SimSxSeyTez WHEREIN M IS 9.3 + OR - 0.3, X+Y+Z 4, X IS 0 TO 3.2, AND Y AND Z ARE 0 TO 4, HAVING A CRYSTAL STRUCTURE OF CUBIC SYMMETRY WHEN THE CHALCOGEN IS Se or S/Se and a closely related crystal structure of pseudocubic symmetry when one of the chalcogens is Te. Process for the preparation of the above chalcogenides comprising heating for at least about 15 minutes at about 800* C. to 1,600* C. and at least about 30 to 65 kilobars pressure the elementary components, mono- and dichalcogenides of silicon, and the binary and ternary chalcogenides of this invention. The dichalcogenides of this invention are useful as semiconductors and in detecting heat and infrared radiation.

it m W i 3 it iii States atet [72] Inventor Paul C. Donohue Wilmington, Del. [21] App]. No. 15,888 [22] Filed Mar. 2,1970 [45] Patented Oct. 26, 1971 73] Assignee E. l. du Pont dc Nemours and Company Wilmington, Del.

[54] SllLlltION CIHIALCOGIENIIDES 11 Claims, No Drawings [52] U.S. C11 23/315, 23/204 R, 252/623 [51] Int. Cl C0lb 33/00, BOlj 17/00, C04b 35/00 Field of Search 23/315, 134,204 R; 252/623 S; 250/205, 206; 148/16 [56] References Cited UNITED STATES PATENTS 3,321,326 5/1967 Young 23/134 Primary Examiner-Oscar R. Vertiz Assistant Examiner-Hoke S. Miller Attorney-James H. Ryan ABSTRACT: Binary, ternary and quaternary silicon chalcogenides of the formula Sim S Se Te wherein m is 93:0.3, x+y+z=4, x is to 0 to 3.2, and y and z are 0 to 4, having a crystal structure of cubic symmetry when the chalcogen is Se or S/Se and a closely related crystal structure of pseudocubic symmetry when one of the chalcogens is Te. Process for the preparation of the above chalcogenides comprising heating for at least about 15 minutes at about 800 C. to 1600 C. and at least about 30 to kilobars pressure the elementary components, monoand dichalcogenides of silicon, and the binary and ternary chalcogenides of this invention. The dichalcogenides of this invention are useful as semiconductors and in detecting heat and infrared radiation.

SILICON CI-IALCOGENIDES BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to new silicon chalcogenides and their preparation.

2. Description ofthe Prior Art Binary sulfides, selenides and tellurides of silicon, e.g., SiTe, SiS SiSe and SiTe containing one atom of silicon per one or two atoms of chalcogen have been described in the literature. With the exception of a new tetragonal, high-pressure form of SiS recently prepared by H. S. Young, US. Pat. No. 3,321,326, and perhaps the little known SiTe, the silicon chalcogenides of the art, are hydrolytically unstable and hence of limited utility, though stable in dry air.

SUMMARY OF THE INVENTION According to the present invention there are provided novel silicon chalcogenides of the formula Si,,,S ,.Se,,Te wherein m is 9.3:03, x+y+z=4, x is to 3.2, y and z are 0 to 4, these chalcogenides having a crystal structure ofcubic symmetry when the chalcogen is Se or S/Se and a closely related crystal structure of pseudocubic symmetry when one of the chalcogens is Te.

Further, according to the present invention there is provided a process for the preparation of the above chalcogenides comprising heating for at least about minutes at about 800-l,600 C. and about 30-65 kilobars pressure in the approximate proportions required to give the desired stoichiometry, combinations of reactants selected from the group consisting of elementary silicon, elementary sulfur, elementary selenium, elementary tellurium, monochalcogenides of silicon, dichalcogenides of silicon, and the binary and ternary silicon chalcogenides of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention provides new crystalline silicon chalcogenides, useful as semiconductors and photoconductors, which contain more atoms of silicon than chalcogen in the unit cell and which exhibit surprising hydrolytic stability, resistin g even concentrated hydrochloric acid.

These new silicon chalcogenides comprise silicon chemically combined with at least one chalcogen of the group S, Se,

delcorn, Non-Stoichiometric Compounds," Academic Press, New York (1964), pp. 98-209. The formulas assigned to the products of this invention are based on density measurements, X-ray studies with space group considerations, and chemical analyses. Of these methods, chemical analysis is considered to be the least reliable due to difficulty in completely removing unreacted starting material and in determining chalcogen when more than one of these elements is present.

As is well known, the relationship between density and molecular weight for crystals ofcubic symmetry is:

l.6604 (M. W.)/a" density where M. W. is the formula weight of the unit cell and a is the unit cell dimension in Angstrom units. Thus, for the product of example 1, below, which had measured density and cubic cell dimension of 3.620 g./cm. and 10.229 A., respectively, the formula weight of the unit cell is calculated to be 2,333.6. Analytical data show an atomic ratio of Si to Se of approximately l.56/0.65, and space group considerations show that the number of silicon and selenium atoms in the cell must each be an even whole number. Hence, the unit cell formula weight of 2,333.6 corresponds approximately to Si Se or to a formula unit of Si -Se The density calculated for a unit cell of four Si Se formula units with a=l0,229 A. is 3.616 g./cm. which is close to the measured 3.620 g./cm. Similar calculations for example 2 show asilicon/selenium atomic ratio of approximately l.5l/ 0.698 and a calculated unit cell molecular weight based on measured density and X-ray data of 2,337.7, corresponding to si se or to four formula units of Si se Theoretical density calculated for a unit cell of four Si Se, formula units in which a=l0.235 A. is 3.607 g./cm. as compared to the measured 3.62 g./cm. The atomic ratio of seleniurn to silicon is:

From Analytical Data From Density and X-Ray Data 0.420

Example I Example 2 TABLE 1 Example N 0.

Unit cell molecular weight 1 Atomic ratio of tellurium to silicon Cale. from analytical data from density and X-ray data Formula unit 1 Observed 1 Calculated from density and X-ray data, taking analytical data into consideration.

9 Onc cighth unit cell molecular weight. 3 Based on measured cell dimension.

and Te, wherein the atomic ratio of chalcogen to silicon is 0.430i0.0l4, said chalcogenides containing at most 0.355 atom of S per atom of Si, and having a crystal structure of cubic symmetry when the chalcogen is Se or S/Se and a closely related crystal structure of pseudocubic symmetry when one of the chalcogens is tellurium. The products may also be described as binary, ternary, and quaternary silicon chalcogenides of the formula Si,,,S,Se,,Te, wherein m is 9.3:03, x+y+z=4, x is 0 to 3.2, and y and z are 0 to 4. Preferred compositions have the formula Si S,Se,Te, wherein x, y, and z are as defined above.

Deviation from exact stoichiometry frequently occurs in chalcogenidessee, for example, Wadsleys chapter in Mansize by their presence. In any event, density measurements are remarkably consistent.

The silicon selenides and the silicon sulfide-selenides of this invention have cubic crystal symmetry with cubic cell dimen-' sions of 10.201003 A. In contrast, the silicon tellurides, silicon sulfide-tellurides, and the silicon sulfide-selenide-tellurides possess pseudocubic crystal symmetry requiring monoclinic indexing in which a=l4.72i0.l2 A., b=l4.6l:': 0.18 A., FIO.39ZO.Q8 A., and B=90.3 Il:t0.07, or on the basis ofa pseudocubic unit cell, a=10.3710.09 A. 0.09

The silicon chalcogenides of this invention may be prepared by heating for at least about minutes (2 hours usually suffices to complete the reaction, however, longer reaction times may also be employed) at about 8001,600 C. and at about 30-65 kilobars (kb.) pressure iii the approximate proportions required to give the desired stoichiometry, combinations of reactants selected from elementary silicon, elementary sulfur, elementary selenium, elementary tellurium, monochalcogenides of silicon, dichalcogenides of silicon, and the binary and ternary products of this invention. These reactions can be represented as:

Si+ one or more silicon monochalcogenides+Si,,,(S,Se,Te).,

Si+ one or more silicon dichalcogenides Si,,,(S,Se,Te) Si,,,Se.,+Si,,,Te Si,,,(Se,Te Si,,,(Se,Te),-+-SiS,-- Si,,,(S,Se,Te) where m and the limiting sulfur content are as defined earlier. While the reactants may be employed in the exact porportions desired in the final composition, it is preferred to use an excess, e.g., 4-50 percent by weight, of chalcogens, which melt at a relatively low temperature and provide a more fluid reaction mixture conducive to formation of crystals. Formation of crystals may also be promoted by prolonging the period at 800-l,000 C. when reaction is effected at this temperature or, in case of higher reaction temperature, by cooling over a period of several hours to about 800l ,100 C. prior to.

quenching to room temperature. Quenching to approximately roomtemperature is preferred, though not essential, since it is helpful in maintaining the phase stable at high temperature. Protracted periods at high temperature are not essential and, if desired, the temperature of the reaction mixture may be dropped abruptly to room temperature, tag, in a few minutes, soon after reaching maximum temperature. Pressure is maintained during the period at elevated temperature and, for convenience, is normally maintained until the charge reaches room temperature.

Temperature and pressure are interrelated. At 800l,000 C. pressures of 3045 kb. usually suffice, whereas at l,000 C. and above pressures of 45-65 kb. or higher are preferred.

Ordinary, commercially available forms of silicon, sulfur, selenium, and tellurium may be employed as reactants. Due to difficulty in purifying the products, high purity reactants are preferred, e.g., 99.999 percent silicon needles and 99.999 per cent sulfur powder, both available from Electron Space Products, Inc. and 99.999 percent grades of selenium and tellurium obtainable from the American Smelting and Refining Company.

Reactants for subsequent reaction at high pressure, e.g., in the tetrahedral anvil press, are usually pulverized and intimately mixed, e.g., by grinding them together using an agate mortar and pestle or by other appropriate means, and then compressed into pellets at about 40 tons/sq.in. pressure employing a conventional pellet press.

The high pressures and temperatures necessary in these reactions may be obtained using a tetrahedral anvil pressure device as described by E. C. Lloyd, et al., .lour. of Research, National Bureau of Standards 63C, 59 (1959). in the device, the prepelleted reactants were placed in a boron nitride container which fitted into a graphite sleeve that served as a resistance heater. This assembly was enclosed in a pyrophyllite tetrahedron and placed in the anvil device, capable ofgenerating pressures in excess of 65 kb.

Four of the calibration points, taken at room temperature, used to determine pressure developed in this device appear in the 1963 Edition of the American Institute of Physics Handbook, pp. 4-43, as follows:

Bismuth l ll 253710.02 kb. Bismuth lI- lll 269610.18 kb. Thallium II llI 36.69101 1 kb.

Barium ll lll 59.0110 kb.

All compressions were made on the cold assembly, and the charges were then heated to the desired temperature using an appropriate thermocouple. No pressure correction for thermocouple behavior has been introduced; standard EMF tables for 1 atmosphere were used. The temperatures were measured at the central surface of the boron nitride crucible. The temperature at the ends of the crucible was probably several hundred degrees cooler.

The silicon chalcogenides are obtained as a single phase or with discrete portions of the reaction product visually distinct from unreacted starting material and byproducts. Segregation is probably the result of a temperature gradient within the reaction mass, for the ends of the pellet may be l00200 C. lower in temperature than the center portion. This usually results in the formation of crystals of the product at the ends of the pellet and congregation of byproducts in the center portion. This distribution permits manual separation of the silicon chalcogenides from impurities. Further purification may sometimes be achieved by washing with 6-l2N hydrochloric acid which, when silicon dichalcogenides are present as byproducts, results in the evolution of hydrogen chalcogenide and formation of colloidal silica that may be easily washed out. Impurities such as uncombined silicon, selenium, and tellurium, while not detectable microscopically, and seldom by X-ray methods, may persist in small quantity making analytical data less reliable than density and Debye-Scherrer X-ray data.

Dependent upon reaction conditions, the products of the invention may be obtained as small crystals or in the form of dark-colored, rodlike single crystals with lengths ranging up to about 2 mm.

Unless otherwise described, it is to be understood in the examples that follow that all parts and percentages are by weight and that reaction products were separated from the boron nitride crucible, crushed to a limited extent, and separated manually from byproducts as completely as possible. Hydrochloric acid (6-l2 normal) was then added (in more than one increment if necessary) until evolution of volatile hydrogen chalcogenide, formed by reaction with byproduct silicon dichalcogenide, ceased. The undissolved residue, consisting largely of Si (S,Se,Te) was washed with water, frequently followed by acetone, and dried in air. The dry product was examined microscopically, and the best crystals were selected under the microscope for X-ray examination and other characterization tests.

Pressure was maintained in the tetrahedral anvil device during reaction, the intermediate cooling stage, if any, and quenching.

EXAMPLE 1 A pellet weighing 0.4333 g. was made from a mixture of 0.7896 g. of Se and 0.5617 g. of Si in which the atomic ratio of Si:Se was 2: 1. It was pressured to 65 kb. and reacted at 1,200 C. for 2 hours, cooled over a 2-hour period to l,l00 C., and quenched while maintaining the pressure. The product was composed of black shiny crystals at the ends of the pellet and black-gray material in the center. The pellet was washed in 6N HCl, and bubbles of H Se were evolved. The crystals themselves were stable in hydrochloric acid. The Debye-Scherrer X-ray diffraction pattern of the crystals showed a pattern which could be indexed on the basis ofa cubic unit cell dimension, a=l0.22910.003 A. (table ll). The density ofthe crystals was measured: Found, 3.620 g./cm. calculated for 4(Si,, -,Se 3.616 g./cm. Differential thermal analysis showed that the crystals decomposed at 627 C. By microchemical analysis the crystals consisted of 43.86% Si and 51.41% Se, i.e., an atomic ratio of Se:Si of 0.417. The space group of the crystals was determined by single crystal analysis using Buerger precession camera methods, and three space groups were found to be possible: [23, [2,3, or lm3. The electrical resistivity of an individual crystal was measured. The material showed semiconducting behavior with a room temperature resistivity of l0 TABLE II Powder Diffraction Pattern of Si Se,

(l intensity ot'X-ray line relative to intensity of strongest line taken as 100; D lattice line spacing) 1 Ir k I D (Obs) EXAMPLE 2 A pellet weighing 0.4052 g. was made from a mixture of 0.4737 g. of Se and 0.2808 g. of Si. It was cold-pressured to 65 ltb. and reacted at l,300 C. for 2 hours, cooled over a period of 3 hours to l,000 C., and quenched. Black shiny crystals were formed at the ends of the pellet, some measuring 2 mm. in length. The crystals were washed with hydrochloric acid to remove SiSe The Debye-Scherrer pattern was similar to that of example 1, and the cubic unit cell was refined by least squares technique to a=l0.235:0.003 A. The density measured on single crystals was 3.62 g./cm. calculated for 4(Si Se 3.607 g./cm. Analysis of the crystals was: Found: 55.14% Se, 42.34% Si. Atomic ratio of Se:Si=0.463; calculated for Si Se 54.20% Se, 45.79% Si. It is believed that the density and X-ray data are more reliable indicators of true stoichiometry than the actual analytical data. Electrical resistivity measurements were made on an individual crystal as before, and semiconducting behavior was observed: p o =2 X10 ohm-cm, Ea=1.0 ev. Photoelectric behavior was not observed.

EXAMPLE 3 A pellet weighing 0.3942 g., made from a mixture of 0.7896 g. ofSe and 0.560 g. of Si, was pressured to 45 kb. and reacted at 1,200C., 45 kb. for 1 hour, cooled during 1 hour to l,000 C., and quenched. The product contained black shiny crystals at the ends of the pellet and black-gray material in the center. The Debye-Scherrer pattern of the 6N HCl-washed crystals showed the same cubic pattern found for the previous examples.

EXAMPLE 4 Si Se,

A pellet weighing 0.4038 g. was made from a mixture of 0.7896 g. of Se and 0.5617 g. of Si. It was pressured to 30 kb. and reacted at 1,000 C. for 2 hours, and quenched. The product was washed with 1:1 concentrated HCkH O to remove byproducts, and a dark material remained. The Debye-Scherrer pattern of the material showed the same cubic phase of Si -,Se., and some excess Si.

EXAMPLE 5 Si Te,

A pellet weighing 0.5266 g. made from a mixture of 1.5312 g. of Te and 0.5617 g. of Si, was pressured to 65 ltb. and reacted at 1,400 C. for 2 hours, cooled for 3 hours to 1,100 C., and quenched. The product contained black crystalline regions at the ends of the pellet and a black-gray material in the center which were separated manually. The Debye-Scherrer pattern of the crystals was similar in some respects to that found in the case of the selenide. It could be indexed on the basis ofa pseudocubic unit cell with a=10.461 A. Guinier data showed line splitting requiring a monoclinic indexing: a=l4.831 A., b=l4.775 A., c=10.460 A., B=90.29 (table 111). Single crystal determination of the space group showed the possibilities Cc or C2/c, both requiring the formula to be a multiple of 4. The density measured on the single crystals was 4.432 g./cm. calculated for 8(Si Te 4.50 g./cm. By differential thermal analysis, the crystals were stable up to 755 C. in air. Electrical measurements made on an individual crystal showed semiconducting behavior. The material had a room temperature resistivity of 400 ohm-cm. and an activation energy Ea-0.33 ev.

TABLE 111 Powder Diffraction Pattern ofSi Te,

TAB LE 11 I Continued Powder Diffraction Pattern of Si; ,Te,,

1 it it 1 D (Obs) D (Cale) 1= intensity oi'X-ray line relative to intensity of strongest line taken as 100. D lattice line spacing. 4" 9 EXAMPLE 6 A pellet weighing 0.5327 g. was pressed from a mixture of 0.7656 g. of Te and 0.2808 g. of Si, pressured to 65 kb. and reacted at 1,300 C. for 2 hours, cooled over a period of 3 hours to 1,000 C. and quenched. The product contained black crystals at the ends of the pellet which were manually separated and washed with concentrated hydrochloric acid. Guinier camera data were taken of the crystals, and the monoclinic unit cell was refined by least squares technique to the values a=14.834 A., b=14.779 A., c=10.457 A., B=90.27. The density measured by displacement technique in bromoform was 4.44 g./cm.. Chemical analysis of the black crystals showed a silicon content of 35.16 percent and a tellurium content of 64.48 percent. The density calculated for 8(S19.5TC4)=4.502 g./cm. The atomic ratio of Te/Si=0.4036 by analysis.

EXAMPLE 7 Si Te A pellet weighing 0.5277 g. made from 0.7632 g. of Te and 0.2808 g. of Si was pressured to 65 kb. and reacted at 1,300 C. for 2 hours, cooled over a period of 2 hours to 1,000 C. and quenched. Large crystals found at the ends of the pellet were washed in 6N hydrochloric acid and in soapy water. The center portion of the pellet was discarded. The large crystals gave a Debye-Scherrer pattern similar to that of the products of examples 5 and 6. The density of an individual crystal was 4.44 g./cm. Monoclinic unit cell dimensions were found to be: a=14.830 A., b=l4.785 A.,c=10.460 A., ,B=90.24. The density calculated for 8(Si Te,) is 4.50 g./cm.". The results of chemical analysis were: Found, 64.01% Te, 34.68% Si, atomic ratio of TezSi, 0.406; calculated for Si Te 65.67% Te, 34.32% Si, atomic ratio ofTe:Si, 0.421.

EXAMPLE 8 A pellet weighing 0.5052 g. was prepared from a mixture of 0.638 g. of Te and 0.281 g. of Si. It was pressured to 30 kb. and reacted at 1,200 C. for 1 hour, cooled during 2 hours to 900 C., and quenched. The produce was composed of black shiny crystalline end regions that were separated manually and a gray center region. The Debye-Scherrer pattern of the crystals showed the presence of Si Te, phase and excess Si.

EXAMPLE 9 EXAMPLE 10 Si (Se, Te),

A pellet weighing 0.4455 g. was made from a mixture of 0.561 g. of Si, 0.638 g. of Te, and 0.395 g. of Se. it was pressured to 65 kb. and reacted at 1,200 C. for 2 hours, cooled for 2 hours to l,000 C., and quenched. The product contained crystalline regions at the ends of the pellet and a black-gray region in the center. The entire lightly crushed pellet was treated with 6N HCl, dried, and the crystals were then separated. The Debye-Scherrer pattern of the crystals showed similarity to those of the binary compounds and was indexed on the basis of a monoclinic unit cell: a=14.612 A., b=14.440 A., c=l0.316 A., ,B=90.37, or a pseudocubic cell, a=10.301 A., By analysis, the crystals consisted of 39.65% Si, 23.99% Se, and 35.18% Te, totaling 98.82%. This corresponds to a composition of Si ,,Se Te, or Si Se, Te, The resistivity of a single crystal was measured. The material showed semiconducting behavior with a room temperature resistivity of 2X10" ohm-cm., Ea=0.7 ev. It was photoconductive with a resistance in the dark to resistance in light ratio ofmore than and appeared to be most sensitive to visible light.

EXAMPLE 1 1 9.s 1.44 -2.se

A pellet weighing 0.5142 g. was pressed from a mixture of 0.1184 g. of Se, 0.5742 g. of Te, and 0.2808 g. of Si. It was pressured to 65 kb. and reacted at 1,200" C. for 2 hours, and quenched. The product contained a shiny silvery crystalline end region, which was separated manually. This end region gave a Debye-Schcrrer pattern which could be indexed on the basis ofa pseudocubic cell, a=l0.380 A. By assuming the applicability of Vegards rule, the composition is Si Se Je 9 EXAMPLE l2 EXAMPLE l3 as isa me A pellet weighing 0.3443 g. was made from a mixture of 0.56 l 6 g. ofSi, 0.4737 g. of Se, and 0.1684 g. ofS. It was pressured to 65 kb. and reacted at l,200 C. for 2 hours, cooled over a period of 2 hours to 1,000 C., and quenched. The product was washed with 12N l-lCl to remove volatile and easily suspended impurities. The center region of the pellet consisted of dark red crystals which showed a Debye-Scherrer pattern having a cubic unit cell, a=l0.l73 Density measured on the crystals was 3.15 g./cm. Assuming the formula Si S Se the value of x=l.68 and hence the formula as rsas zaz may be calculated from the density. Electrical measurements made on a single crystal showed semiconducting behavior. The room temperature resistivity was 4 10 ohm-cm. and Ea=0.7 ev.

EXAMPLE l4 A pellet weighing 0.3421 g. was made from a mixture of Si, Se, and S mixed in the atomic ratio 220.5105, respectively. The pellet was pressured to 65 kb. and reacted at 1,100 C. for 1 hour. cooled over a 3-hour period to 800 C., and quenched. The product was purified by washing with 1:1 concentrated HCl:l-l O, leaving black crystalline and black glassy material. The Debye-Scherrer patterns of both materials were taken and identified. The black crystals showed the pattern of Si; the black glassy phase showed a pattern similar to Si Se but shifted in size. The pattern was indexed on the basis of a cubic unit cell with a=10.182 A. From the composition and mea' sured cell dimension of example 13, using Vegards law, the cell dimension of nonexistent Si s, was calculated to be a=l0.094 A Using this value and the actual value of 10.182 A for the cubic unit cell, the composition of the product was calculated to be Si S Se EXAMPLE 15 as sz oa A pellet weighing 0.3645 g. was made from a mixture of Si, Se, and S, which were ground together in the atomic ratio 2:0.66:0.33, respectively. It was pressured to 65 kb. and reacted at 1250 C. for 1 hour, cooled over a period of 3 hours to l.100 C., and quenched. The product consisted of black microcrystals and reddish rodlike crystals, both of which were washed with 6N l-lCl, water, and acetone. The black microcrystals were identified from the Debye-Scherrer pattern as silicon. The reddish crystals showed a Debye-Scherrer pattern similar to that of Si Se., and was indexed on the basis ofa cubic unit cell with a==l0. 121 A. Assuming the applicability of Vegard's rule in the manner described in example 14, the value ofx is 3.2, and the formula is Si S Se EXAMPLE 16 A pellet made from a mixture of0.56l6 g. of Si, 0.6385 g. of Te, and 0.160 g. of S was pressured to 65 kb. and reacted at 1,200 C for 2 hours, cooled over a period of 2 hours to l,000 C. and quenched. The product consisted of black crystalline material at the ends of the pellet and larger red crystals toward the center. These were separated and washed with 6N HCl, water, and finally acetone. The Debye-Scherrer patterns of both regions showed a similarity to that of the phase Si Te,. The pseudocubic unit cell dimension refined by the least squares technique was a=l0.3767i0.0007 A. Chemical analysis showed 5, 3.61 percent, indicating that the value ofx was approximately 0.8 and hence the formula Si S Te Resistivity measurements on an individual crystal showed semiconductivity with Ea=0.5 ev. The material was mildly photoconductive with a ratio ofdark to light resistance of4.5.

EXAMPLE 17 A pellet weighing 0.3493 g. was pressed from a mixture of 0.144 g. ofS, 0.1914 g. ofTe, and 0.2808 g. of Si. lt was pressured to 65 kb. and reacted at l,200 C. for 2 hours, and quenched. The product consisted ofa mixture of phases. Soluble material was removed by washing with 12 N HCl. The Debye-Scherrer pattern of the remainder showedthe presence of excess Si and the Si,, Te,-type phase. It could be indexed on the basis of a pseudocubic unit cell a=l0.3l9 A. Using Vegards rule, the value of x is approximately 1.4, i.e., B.5 1.4 2.6-

EXAMPLE l8 A pellet weighing 0.4084 g. was pressed from a mixture of 0.5617 g. of Si, 0.1069 g. ofS, 0.4253 g. of Te, and 0.2632 g. of Se. It was pressured to 65 kb. and reacted at 1,200C. for 2 hours, and quenched. The product contained several regions.

Soluble material was washed out with 12N HCl. The remain- EXAMPLE A Crystalline samples of Si Se, (example 1) and Si Te, (example 6) were alternately utilized in a device to detect heat and infrared radiation. Each was placed in turn in a DC circuit in series with an ohm-meter. Heat from a soldering iron and light from an incandescent lamp, filtered to exclude short wavelength light and to pass infrared radiation, were in turn brought near to the crystals. The amount of heat from the soldering iron and infrared radiation from the lamp was detected by measuring the change in current flow with the ohm-meter. For Si Se the resistance changed from 3X10 ohm-cm. to 2 l0 ohm-cm. upon infrared irradiation and to 2X10" ohmcm. with heat. For Si Te the resistance changed from 6X10 ohm-cm. to 4X10 ohm-cm. in heat.

The silicon chalcogenides of this invention are also useful as semiconductors in a variety of other applications. They may be doped to impart n and p conductivity and used as rectifiers when reverse biased or as radiation emitters when forward biased. Electrical devices may be prepared by depositing active elements on substrates of the crystalline chalcogenides. Utility is greatly increased by the usual hydrolytic and thermal stability of the materials.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. Silicon chalcogenides of the formula Si S Sefle, wherein m is 9.31-0.3,x+y+z=4,x is 0 to 3.2, y and z are 0 to 4, said chalcogenides having a crystal structure of cubic symmetry when the chalcogen is Se or S/Se and a closely related crystal structure of pseudocubic symmetry when one of the chalcogens is Te.

2. The silicon chalcogenides of claim 1 wherein m is 9.5.

3. The silicon chalcogenide ofclaim 2 wherein x and z are 0 and y is 4; Si ,,Se.,.

4. The silicon chalcogenide of claim 2 wherein x and y are 0 and z is 4; Si Te 5. The silicon dichalcogenide of claim 2 wherein x is 0, y is 1.44 and z is 2.56; Si Se Te 6. The silicon dichalcogenide of claim 2 wherein x is 0, y is 9. The silicon dichalcogenide of claim 2 wherein x is 3.2, y is 3.6 and z is 0.4; Si ,Se Te 0.8 and z is 0; Si S Se 7. The silicon dichalcogenide of claim 2 wherein x is 1.68, y 10. The silicon dichalcogenide of claim'2 wherein x is 0.8, y is 2.32 and z is 0; Si S Se is 0 and z is 3.2; Si,, S Te 8. The silicon dichalcogenide of claim 2 wherein x is 1.4, y is 5 11. The silicon dichalcogenide of claim 2 wherein x is 1.4, y 2.6 and z is 0; Si S Se is 0 and z is 2.6; Si S Te 

2. The silicon chalcogenides of claim 1 wherein m is 9.5.
 3. The silicon chalcogenide of claim 2 wherein x and z are 0 and y is 4; Si9.5Se4.
 4. The silicon chalcogenide of claim 2 wherein x and y are 0 and z is 4; Si9.5Te4.
 5. The silicon dichalcogenide of claim 2 wherein x is 0, y is 1.44 and z Is 2.56; Si9.5Se1.44Te2.56.
 6. The silicon dichalcogenide of claim 2 wherein x is 0, y is 3.6 and z is 0.4; Si9.5Se3.6Te0.4.
 7. The silicon dichalcogenide of claim 2 wherein x is 1.68, y is 2.32 and z is 0; Si9.5S1.68Se2.32.
 8. The silicon dichalcogenide of claim 2 wherein x is 1.4, y is 2.6 and z is 0; Si9.5S1.4Se2.6.
 9. The silicon dichalcogenide of claim 2 wherein x is 3.2, y is 0.8 and z is 0; Si9.5S3.2Se0.8.
 10. The silicon dichalcogenide of claim 2 wherein x is 0.8, y is 0 and z is 3.2; Si9.5S0.8Te3.2.
 11. The silicon dichalcogenide of claim 2 wherein x is 1.4, y is 0 and z is 2.6; Si9.5S1.4Te2.6. 